Plant hormones and growth in plants

What are tropisms?

Directional growth responses that plants carry out in reaction to specific stimuli
- Light
- Gravity

What are some abiotic stressed plants have?

- Lack of water

- Herbivory

- Competition

- Light pH and temperature changes

Important plant hormones

- Auxins

-Gibberellins

- Abscisic acid (ABA)

- Ethene

Auxin's roles in plants

- Controls cell elongation
- Growth stimulants
- Prevents abscission
- Maintains apical dominance
- Tropisms
- Stimulates ethene release
- Fruits ripening

Gibberellin's roles in plants

- Stem elongation

- Triggers food store mobilisation at germination

- Stimulates pollen tube growth in fertilisation

Ethene's roles in plants

- Ripens fruits

- Promotes abscission in deciduous trees

ABA's roles in roles

- Maintains dormancy of seeds and buds

- Stimulates cold protective responses
(antifreeze production, stomatal closing)

- Interferes with the action of
Gibberellin

What is abscission?

Dropping of leaves, flowers, or fruit

What do plants produce chemicals for?

- Signal to other species to protect against insects

- Defences against herbivores

What do plant hormones control?

Growth of plants
- Germination of the seed
- Growth of a tree

What is germination?

The process by which a plant grows from a seed

Outline the process of germination

- Seed absorbs water

- Embryo activated and produces gibberellins

- Gs stimulate enzyme making (breaks down food stores in seed)

- Embryo plant uses stores to make ATP

- The plant can grow and break out the seed coat

Where are the food stores in dicot seeds?

Cotyledon

Where are the food stores in monocot seeds?

Endosperm

What are the digestive enzymes needs for germination?

Amylase and protease

How are amylase and protease made for germination?

Gibberellins switch in genes that code for them

The effect of gibberellins on the synthesis of amylase in isolated tissue from barley seeds

What affects whether a seed will germinate?

Gibberellins

Abscisic acid (Gs antagonist)

What are some experimental evidence of the roll of gibberellins in germination?

1. Seeds have been made without gibberellins
2. Gibberellin biosynthesis inhibitors

* Seeds don't germinate

Role of gibberellins in germination

What do auxins do?

Cell elongation

Where are auxins made?

Tips of roots and shoots

Meristems

What direction do auxins move in a plant?

Down the stem and up the root in transport tissue from cell to cell

What does the effect of auxins depend on?

- Auxin concentration

- Interactions with other hormones

What effects do auxins have on plant growth?

- Stimulate growth of apical shoot

- High concs suppress growth of lateral shoots

- Low conc promote root growth

How do auxins stimulate the growth of the main apical shoot?

* Auxin binds to specific receptor sites in the plant cell membrane
* pH falls to 5 (op pH for enzymes to keep walls flexible and plastic)
* Cells mature, auxin is destroyed, pH rises, enzymes are inactive, walls are rigid no growth

The effect of auxin on apical shoot growth

Graph to show the effect of an external application of auxin on pH levels in the cell walls and on shoot growth

How do high concs of auxins suppress the growth of the lateral shoots?

- Results in apical dominance

- Lateral shoots are inhibited at the tip

- Further down conc is low so lateral shoots are stimulated

What is apical dominance?

Suppression of the growth of lateral buds by hormones produced by the apical meristem.

Apical dominance affects the form of a plant and can be clearly seen in conifers

How do low concs of auxins promote root growth?

- If apical shoot is removed the amount of auxin reaching roots is decreased

- Root growth slows

- High concs inhibit root growth

Effect of auxin on roots, lateral and apical buds

Auxin can inhibit and stimulate
- Highest conc apical bud is most stimulated
- No cell elongation
- Further down stem lateral bud is stimulated
- Cell elongation
- Lowest conc promotes root growth

What do gibberellins affect?

Length of internodes
- regions between leaves on a stem